Well so far I've dug into three stories: Love Roma, Vagabond, and Majime na Jiken.
I've read Majime na Jikan too, it was pretty neat. If you want another good romance story, try Tonari no Kaibutsu-kun. It's about a girl who wants to do nothing but study and a delinquent sort of guy. They don't get into sex, but it's one of the best shoujo manga I've read. It's 48 chapters and complete.
Another good romance story is Horimiya, which is a shonen manga (see how demographics don't really tell you anything about genres?). It's about a popular girl who is basically a homemaker when she's not at school and a nerd who has tattoos and piercings. It's still ongoing but it's at 38 chapters.
If you want some action, try Spirit Circle. It's a guy who goes through his past lives. It's at about 27 chapters and ongoing. If you have a crunchyroll subscription, you can read it all there.
Another action series by the same guy is Lucifer and the Biscuit Hammer. It's about a group of people who have to save the world from being destroyed by a giant hammer. It's 56 chapters and complete.
Also have you read or watched Full Metal Alchemist? If you haven't, do that.
edit: Also, Koe no Katachi. It's about deafness and bullying. It's ongoing with 42 chapters, but it's probably going to end soon.
Also, there are monthly series (have more pages, around 30 to 40) and weekly series (around 18-22 pages). Everything I listed is monthly except for Koe no Katachi. Horimiya is monthly, but it has a lot of shorter 15 paged chapters, so it's not as long.
edit2: Also, Hi Score Girl is a really short, but cute romance manga about a kid who loves arcades and the girl who keeps bothering him at them.